ZIF-8-Based Membranes for Carbon Dioxide Capture and Separation

Nov 3, 2017 - He is currently a lecturer in the key laboratory of polymer processing engineering of the Ministry of Education, South China University ...
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ZIF-8-Based Membranes for Carbon Dioxide Capture and Separation Xiao Gong,*,† Yongjin Wang,‡ and Tairong Kuang*,§ †

State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, No. 122, Luoshi Road, Wuhan 430070, China ‡ Dalian Institute of Chemical Physics, Chinese Academy of Science, 457 Zhongshan Road, Dalian 116023, China § The Key Laboratory of Polymer Processing Engineering of Ministry of Education, South China University of Technology, No. 381, Wushan Rd., Tianhe, Guangzhou 510640, China ABSTRACT: Zeolitic imidazolate framework-8 (ZIF-8) which is a class of metal−organic frameworks (MOF) is a newly synthesized porous material. It presents thermally and chemically stable properties for the application of carbon dioxide (CO2) adsorption/separation due to its porous structure. In this review, we briefly summarized the most recent studies on ZIF-8-based membranes for CO2 adsorption and separation. We also discussed the ZIF-8 fabrication methods and different parameters such as composite materials and temperature and pressure effects on the permeability and selectivity of CO2. Moreover, the special features of selected adsorbed and separated membranes were discussed. Lastly, future research directions and challenges were briefly discussed. KEYWORDS: ZIF-8, CO2, Membranes, Capture, Separation



INTRODUCTION Recently, the global carbon dioxide (CO2) levels have approached a worrisome milestone.1,2 It is very important and necessary to solve this global issue of CO2 pollution since it may lead to many environmental problems such as greenhouse effect, climate changes, and snow cover melts. So far, many techniques have been developed to solve this problem including physical/chemical adsorption, cryogenic distillation, temperature/pressure-swing adsorption, membrane separation technology, and so on.3−5 Among these technologies, the membrane separation method, especially metal−organic frameworks (MOFs) composite membrane technology, is highly attractive and has been reported recently6 because of its promising advantages such as low energy consumption, high economic benefit, easy industrialization, and high efficiency. In 2006, Yaghi et al.7 reported that they had successfully synthesized a series of crystals, named zeolitic imidazolate frameworks (ZIFs), with metal ions (Zn or Co) and imidazolate-type links.7 ZIFs have been gaining particular attention as one of the most investigated MOFs since they are easy to fabricate, present remarkable stability, and have a special molecular sieving effect. ZIF-8 with a single crystal X-ray structure, as shown in Figure 1, is a prototypical member of ZIFs with a SOD (sodalite) zeolite-type structure.8 Though several reviews9−13 introduced the class of MOFs materials and their applications for CO2 adsorption and separation, a more detailed review that focuses on adsorbing and separating CO2 by ZIF-8 nanoparticles is rare. It is very important and necessary to gather attention on this particular © 2017 American Chemical Society

topic since ZIF-8 shows a high porosity, high surface areas, the best thermal stability, outstanding chemical resistance, and strong mechanical strength.7,14−16 Hence, this review aims to introduce the recent advances in ZIF-8-based membranes for applications in CO2 adsorption and separation. Herein, we separately summarized four applications, including pure CO2 adsorption and CO2 separation from methane (CH4), hydrogen (H2), and nitrogen (N2). CO2 Adsorption and Separation. In this review, four technologies, including pure CO2 adsorption and the separations of CO2/CH4, CO2/H2, and CO2/N2 by ZIF-8 porous materials have been summarized. Different parameters such as the size of ZIF-8, composite materials, and temperature and pressure effects on CO2 adsorption and separation performances have also been discussed herein. Pure CO2 Adsorption. ZIF-8 composite membranes are able to adsorb CO2 gas because the crystals within ZIF-8 can provide Langmuir sites which can be replaced by CO2 molecules.17 The loading percentage of ZIF-8 in the membrane can influence the permeability of CO2.18−20 For example, Song et al.18 reported that they have synthesized a membrane by dispersing ZIF-8 nanoparticles into the polymer of Matrimid 5218. Such a membrane can highly increase the permeability of CO2 during the pure gas sorption tests. This is attributed to the special structure of loaded ZIF-8 in the membrane, which can Received: October 7, 2017 Revised: October 29, 2017 Published: November 3, 2017 11204

DOI: 10.1021/acssuschemeng.7b03613 ACS Sustainable Chem. Eng. 2017, 5, 11204−11214

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Figure 1. Single crystal X-ray structures of ZIFs-8. The net is shown as a stick diagram (left) and as a tiling (center). The largest cage in the ZIF-8 is shown with a ZnN4 tetrahedra in blue (right). H atoms are omitted for clarity. (Reprinted with permission from ref 7.)

Figure 2. Irreversible chemical reaction among ZIF-8, water, and CO2, which creates both zinc carbonate (or zinc carbonate hydroxides) and single 2-methylimidazole crystals. (Reprinted with permission from ref 22.)

298 K and 25 bar. Due to the electron-donating effect of amino functional groups, it is expected that amino modification is an efficient method to improve CO2 adsorption of ZIF-8. Therefore, Liu et al.40 simulated the CO2 adsorption capacity of ZIF-8 and revealed that in the lower pressure regime the capacity is ZIF-8 < ZIF-8-NH2 < ZIF-8-(NH2)2, while in the high-pressure regime the capacity is ZIF-8 < ZIF-8-(NH2)2 < ZIF-8-NH2. Since the existence of −NH2 groups can generate new adsorption sites (Figure 3), amino modification of ZIF-8 can obviously enhance its CO2 adsorption capacity. The above experimental and theoretical studies can help researchers better design ZIF-8-based membranes for CO2 adsorption in the future.

provide a much larger free volume than that of neat polymer membranes. They also found that the permeability of CO2 increased with the loading of ZIF-8 in the matrix membranes. The permeability of CO2 reached 28.72 Barrer when the ZIF-8 loading increased up to 30%, which was almost three times that of the 5% ZIF-8 loading. However, the mechanical strength decreased at a very high loading of ZIF-8 (∼80%), and the permeability started to decrease when the loading was higher than 50% for a pure CO2 sorption test.19 ZIF-8 can also be used as be 3D hierarchical structures to improve CO2 adsorption,21 and water can irreversibly react (Figure 2) with CO2 and ZIF-8, leading to a decrease in CO2 adsorption.22 Usually, ZIF-8 is prepared with the composite polymer as a nanoporous membrane for gas adsorption. The performance can be enhanced by this nanoporous membrane compared with neat polymer membranes.15,23,24 Moreover, the membrane can also be prepared on a wide range of substrates such as a copper net,25 alumina porous support,26 titania support,27−29 tubular αalumina support,28,30,31 Zn-related nanofibers,32−35 and Nylon membrane.36 For example, Xu et al.37 prepared a ZIF-8 membrane on alumina hollow fibers using a concentrated ZIF-8 synthesis gel. They found that such a membrane presented a strong adsorption capability of CO2 with a low permeance, which is attributed to the fact that the adsorbed CO2 blocks the pores of ZIF-8, gradually reducing the permeance of CO2. Additionally, ZIF-8 is more thermally stable than the commercial adsorbents such as Zeolite-13X. The activation temperature has almost no influence on the CO2 adsorption capability for ZIF-8.38 The modifications of the postsynthetic method,39 amine-modified method,40,41 and so on42 have been used to improve the BET surface area of ZIF-8 to add a new sorption site of −NH2 groups, which can enhance the adsorption efficiency. In order to improve the adsorption performance toward CO 2 , ZIF-8 was modified by a postsynthetic method using etheylenediamine.39 Results showed that the CO2 adsorption capacity per surface area for the modified ZIF-8 could be almost two times that of ZIF-8 at

Figure 3. Contour maps of the electrostatic potential (ESP) for (a) ZIF-8, (b) ZIF-8-NH2, and (c) ZIF-8-(NH2)2. (Reprinted with permission from ref 40.) 11205

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ACS Sustainable Chemistry & Engineering CO2/CH4 Separation. Natural gas usually contains lots of impurities including CH4, and some other light gases such as C2H6, C3H8, and acid gases such as CO2 and H2S, in which CO2 is regarded as the most crucial and needs to be removed since it can corrode pipelines and reduce the energy content of the natural gas.43 Many technologies have been developed to separate CO2 from natural gas such as chemical adsorption44,45 and cryogenic separation.46 However, these technologies are limited by high loss of solvent, high consumption of energy, and low efficiency during the separation.47,48 In order to solve these problems, the membrane separation method was reported to have a high potential application in commercial CO2/CH4 separation.49−53 Among the separated membranes, the mixed matrix membrane (MMM) was recently the most highly developed due to its outstanding advantages such as high selectivity and good stability. Here, we focus on reviewing this typical membrane, which is composed of ZIF-8 and various polymers. The influence of incorporated polymers,54 ZIF-8 nanoparticles parameters including size,55,56 chemical modification,57,58 structure59,60 and loading percentage54,61−64 and fabrication conditions including temperature,56,65,66 method,67,68 time,69 and so on43,70−73 on the performance of CO2 and CH4 separation are summarized herein. Chi et al.55 investigated the size of the ZIF-8 effect on CO2 capture. The different sizes of ZIF-874,75 with the same surface areas were achieved by controlling the sources of zinc,76,77 and their FE-SEM images are shown in Figure 4. The separation

and larger ZIF-8 nanoparticles, respectively. A little higher performance of medium ZIF-8 may be attributed to the interactions and interfaces of the SEBS/ZIF-8 membrane. The conclusion that CO2/CH4 selectivity changes with ZIF-8 particle sizes which is reported in a previous work56 may be related to the different surface areas of ZIF-8 particles with different sizes. In order to change their properties, ZIF-8 nanoparticles were also ammonia modified to improve the selectivity performance as shown in Figure 5.57,58,80 When MMM was prepared with the ZIF-8 that was modified under a 25 mL ammonium hydroxide solution at 60 °C, it presented a largest selectivity of CO2/CH4.57 The largest BET surface area and the smallest mesopore volume observed on the ammonia-modified ZIF-8 may be attributed to the best separation performance. The loading of ZIF-8 in this membrane is only 0.5%, which is significantly smaller than in previous work due to the asymmetric property61 of the membrane. Before mixing nanoparticles with incorporated polymers, ZIF-8 can also be prepared as special heteronanostructures81 such as hollow zeolite imidazole frameworks59 and mixed-linkers82,83 to generate lower density nanoparticles with a larger BET surface area. With the help of these special structures, the membranes can achieve a similar separation performance with less ZIF-8 loading, which can reduce the cost of the membranes.84,85 Besides changing the properties of ZIF-8 nanoparticles, more modifications can be done when mixing ZIF-8 with incorporated polymers to enhance the separation capability. Bushell et al.54 found that the largest separation factor of CO2/ CH4 could be observed when the loading of ZIF-8 was around 28 vol %. Either smaller or higher loading could reduce the selectivity of CO2/CH4.54,61,86 The fabrication conditions of MMMs such as heat treatment56 and post-treatment65 combined with other MOFs87 fabrication technologies including secondary growth67 and sonication method68 were also investigated for the application in CO2/CH4 separation. For example, Nordin et al.56 reported that ZIF-8 heat treated at 100 °C for a minimum of 12 h could enhance its phase crystallinity, leading to a surface area of 981.1 m2 g−1. When the heat-treated ZIF-8 of the smallest size was incorporated into a polysulfone (PSf) matrix (Figure 6), the MMMs could exhibit CO2/CH4 selectivity of 28.5, which is obviously larger than the 19.43 obtained for the neat PSf membrane. Thompson et al. showed that high-intensity ultrasonication could induce Ostwald ripening on ZIF-8 nanoparticles.68 The ZIF-8-based membranes fabricated by both direct and indirect sonication exhibited good adhesion between the polymer and ZIF-8 nanoparticles. However, membranes prepared by indirect sonication had an agglomeration of nanoparticles, while membranes prepared by direct sonication showed the well dispersion of nanoparticles. Permeation tests revealed the significant improvement in permeability of CO2 and enhancement of CO2/CH4 selectivity in membranes fabricated with high-intensity sonication. The above studies have proven to be straightforward methods to simultaneously improve CO2 permeability and ideal CO2/CH4 selectivity, which are beneficial for future applications in gas separation membranes. CO2/N2 Separation. The capture of CO2 in the atmosphere mostly generated from the burning of fossil fuels is another critical environment problem.88 Since more than two-thirds of fuel gas is nitrogen, many technologies have been developed to separate CO2 from nitrogen-rich streams.89−95 Here, we focus

Figure 4. FE-SEM images of ZIF-8 nanoparticles of different sizes: (a,b) ZIF-8(S), (c,d) ZIF-8(M), and (e,f) ZIF-8(L) (Reprinted with permission from ref 55.)

membranes (polystyrene-block-poly(ethylene-ran-butylene)block-polystyrene (SEBS)/ZIF-8) were finally fabricated by dispersing ZIF-8 into a block copolymer78,79 homogeneously. It is found that the selectivity of CO2 and CH4 can be enhanced with a ZIF-8-added membrane compared with a neat polymer membrane, while the selectivity is almost independent of ZIF-8 particle sizes if the surface area of particles ZIF-8 keeps as the same. The selectivity α is 5.2, 5.4, and 5.2 for small, medium, 11206

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Figure 5. Idealized crystal structure of amine-modified ZIF-8 (adapted from Liu et al.40). NH2−ZIF-8 represents the −NH2 attached on a single side of CC, while (NH2)2−ZIF-8 represents the −NH2 group attached on both sides of CC in the imidazole linkers. (Reprinted with permission from ref 57.)

dispersing into a PSf polymer, to fabricate a PVAm/ZIF-9/PSf MMM, which can further improve the separation performance.99 Similar to CO2 adsorption, ZIF-8 nanoparticles can not only be directly mixed with polymers but also can be grown on different substrates to fabricate the separation membranes. Marti et al.100 fabricated a continuous inorganic membrane with ZIF-8 nanoparticles on a polymeric hollow fiber support by the continuous flow synthesis method as shown in Figure 8. This method is simple, scalable, low cost, and environmentally friendly, and the selectivity of CO2/N2 can be highly enhanced compared to MMMs. Similarly, Isaeva et al.101 used an in situ synthesis method to grow ZIF-8 membranes on polymeric and inorganic supports for the application in CO 2 and N 2 separation. In addition, ZIF-8/polysulfone,102 ZIF-8/(polyether block polyamide with flexible polyether segments (PEBAX)2533),103 ZIF-8/(1,3-di-n-butyl-2-methylimidazolium chloride (DnBMCl)),104 and so on were reported to be used as a separated membrane for CO2 and N2 separation. CO2/H2 Separation. The separation of CO2 and H2 usually needs to treat the products of a water−gas shift reaction in order to obtain high purity hydrogen.105 Similar to CO2/CH4 and CO2/N2 separations, ZIF-8 nanoparticle composite membranes also present very good properties for CO2/H2 separation. The mechanism of pair gas separation includes diffusion selectivity and solubility selectivity.106,107 The kinetic diameters of CH4 and N2 are 3.80 and 3.64 Å, respectively, which are larger than CO2 (3.30 Å), while the kinetic diameter of H2 is 2.89 Å which is smaller than CO2. The diffusion selectivity favors the transport of the smaller molecules of CO2, CO2, and H2 for the separation of CO2/CH4, CO2/N2, and CO2/H2, respectively, and the solubility selectivity favors the sorption of the more condensable gas, which is CO2 for all the separations. Therefore, CO2/H2 separation is more complex and difficult than CO2/CH4 and CO2/N2 separations since the solubility selectivity and the diffusion selectivity of CO2/H2 separation present opposite trends.49,108 Generally, this separation is conducted under high temperature in order to reduce the adsorption of CO2. It was reported that a pure ZIF-8 membrane which was grown on hollow ceramic fiber tube displayed a large H2 permeability and good separation performance.109 Additionally, ZIF-8 deposited with graphene oxide (GO) to fabricate a ZIF8/rGO membrane (Figure 9) showed a high hydrogen selectivity.110−112 ZIF-8 particles in situ crystallized between the sheets of graphene membranes were strongly anchored with graphene by coordination bonds. The effects of mechanical flexibility,113 thermal stability,114 and fabrication methods115 of ZIF-8 membranes on separation performance have also been

Figure 6. Cross-section morphology of the heat-treated ZIF-8 MMM at (a) 100 nm, (b) 300 nm, and (c) 500 nm (Red circles represent the presence of ZIF-8 clusters). (Reprinted with permission from ref 56.)

on summarizing the separation of CO2 and N2 with ZIF-8added membranes.96 Dai et al.97 first successfully fabricated mixed matrix hollow fibers membranes, which were composed from ZIF-8 and Ultem 1000 (a poly(ether imide)),98 by the method of dry jet− wet quenching for the application of CO2/N2 separation. The selectivity was enhanced by as high as 20% for these membranes over pure polymer hollow fibers membranes. They also investigated the temperature and feed pressure effect on the permeance and selectivity as shown in Figure 7. A maximum selectivity of 32 could be achieved under a larger feed pressure (50 psia) and small temperature (25 °C). Moreover, it is noticed that this study is the first time it was reported that a gas separation membrane was extended to an asymmetric membrane. Additionally, ZIF-8 was presynthesized with the polymer of poly(vinylamine) (PVAm), following with 11207

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Figure 7. Mixed gas permeation results using a feed containing 20% CO2 with a N2 balance. (A) Permeance of CO2 (closed symbols) and N2 (open symbols). (B) Permselectivity results for mixed gas measurements. Measurements were conducted at 25 °C (circles), 35 °C (triangles), and 45 °C (squares). (Reprinted with permission from ref 97.)

pores of a polymeric membrane during solvent casting, and these nanoparticles could produce ZIF-8 in situ growth by solvothermal synthesis. This technique leads to mechanically flexible self-supporting ZIF-8 polymeric membranes which could be applied for the synthesis of a variety of MOF systems and serve as a platform for flexible MOF gas separation. James et al. systematically investigated the thermal stability of ZIF-8 membranes which is important for high temperature gas separation applications.114 Results showed that ZIF-8 membranes kept their crystallinity and structural integrity when the temperature was below 150 °C. However, ZIF-8 membranes obviously underwent thermally induced carbonization when the temperature was above 150 °C. Lo et al. demonstrated a novel method using pseudopolymorphic seeding for the rational synthesis of hybrid membranes with ZIFs which can enhance the separation performance of the ZIF-L@ZIF-8 hybrid membranes which may be attributed to the interlayer spacing among ZIF-L crystals allowing for the rapid diffusion of hydrogen.115 Moreover, in order to further improve the selectivity of CO2/ H2, ZIF-8 particles were mixed with polymers to fabricate MMMs for CO2/H2 separation.116−122 For example, SanchezLainez et al.116 prepared a membrane with ZIF-8 and polybenzimidazole (PBI), which increased the selectivity of CO2/H2 nearly by 55% compared to a pure PBI polymer membrane. The relationship between the ZIF-8 particle size and separation performance was discussed. The better performance of larger ZIF-8 particles may be due to the lower degree of agglomeration. They also indicated that although the performance of MMMs with dry ZIF-8s was better, wet ZIF-8s could not be removed since they were much easier to be industrialized. This issue could be reduced or eliminated by increasing the loading percentage of ZIF-8s and by increasing the separation temperature.116,119 Diestel et al. prepared MMMs with bulky ZIF-8 nanoparticles and investigated the permeation behavior of these membranes for the gas mixture H2/CO2.117 They found that the ZIF-8-based membranes obviously improved the hydrogen permeability, while the mixed gas separation factor kept constant at 3.5 ± 0.3. In order to obtain high gas flux and high selectivity, Wijenayake et al.118 showed that cross-linking the surface of the MMM by reacting with ethylenediamine vapor (Figure 10) could yield a

Figure 8. (A) Processing of supported ZIF-8 hollow fiber membrane. (B) Cross-section diagram showing the hollow core, porous Torlon structure, and ZIF-8 on the surface of the support. (Reprinted with permission from ref 100.)

Figure 9. Assembly of a ZIF-8/rGO membrane. (a, b) Synthesis process of the ZIF-8/rGO membrane by in situ crystallization. (c, e) Crystalline structure of rGO and ZIF-8. (d) Coordination bonds between rGO and ZIF-8. Carbon, oxygen, nitrogen, and zinc atoms are shown in gray, red, blue, and yellow, respectively. (Reprinted with permission from ref 111.)

reported. Since mechanical flexible and stable MOF composite membranes are highly desirable, Hess et al. developed a new synthesis method that could form the flexible, noncontinuous ZIF-8−poly(ether sulfone) (PES) composite membranes.113 They showed that ZnO seed nanoparticles could enter into 11208

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Figure 10. Schematic representation of the formation of the cross-linked skin in 33.3 wt % ZIF-8/6FDA-durene MMM upon reaction with ethylenediamine (EDA) vapor. (Reprinted with permission from ref 118.)

Table 1. Permeability and Selectivity of CO2 by Selected ZIF-8-Based Membranes Type of incorporated materials

Fabrication technique

Feature

Permeability/Selectivity of CO2

ref

Matrimid 5218 polymer

Solution mixing

Scale up for industrial applications

18

Alumina hollow fiber PSf

Concentrated synthesis gel Dry/wet phase inversion

Application for CO2 adsorption Asymmetric MMMs

PSf

Dry/wet phase inversion

Ammonia modified ZIF-8

PVC-g-POEM

Solution mixing

Copolymer (SEBS)

Solution mixing

Solvothermal surface coating and selective removal for structure Investigate ZIF-8 particle sizes effect

Permeability: 28.72 Barrer (30 wt % loading, annealed under vacuum at 230 °C for 18 h) Adsorption of CO2 with very low CO2 permeance Selectivity CO2/CH4: 23.16 (0.5 wt % loading, feed pressure 4 bar, 27 °C) Selectivity CO2/CH4: 34.09 (0.5 wt % loading, feed pressure 4 bar, 27 °C) Selectivity CO2/CH4: 12.2 (10 wt % loading, 35 °C)

PIM-1 Poly(ether imide) (Ultem 1000) PSf/PVAm

Casting Dry jet−wet quench method

ZIF-8 loading effect Asymmetric hollow fibers membranes

Solution mixing

PVAm premodifed ZIF-8

Polyimide-amide polymer

Simple, scalable, less cost, and environmentally friendly Pure ZIF-8 membrane

Graphene oxide

Flow synthesis bore/shell reagent flow Crystallizing-rubbing seed deposition Layer-by-layer deposition

PBI

Solution mixing

PBI

Solution mixing

Hollow ceramic fiber

ZIF-8 combine with two-dimensional materials Validation through interlaboratory test CO or water vapor impurity effect

37 61 57 59

Selectivity CO2/CH4: 12.0 ± 0.3 (30 wt % loading, 35 °C) Selectivity CO2/CH4: 18.6 ± 1.9 (28 vol % loading) Selectivity CO2/N2: 32 (13 wt % loading, feed pressure 50 psia, 25 °C) Selectivity CO2/N2: 112 (13.1 wt % loading, feed pressure 0.3 MPa, 22 °C) Selectivity CO2/N2: 52 (25 °C)

100

Selectivity CO2/H2: 5.2 (room temperature)

109

Selectivity CO2/H2: 14.9 (1 bar, 250 °C)

110

Selectivity CO2/H2: 7.5 (20 wt % loading, feed pressure 3 bar, 150 °C) Selectivity CO2/H2: 26.3 (63.6 vol % loading, 0−30 atm, 230 °C)

116

55 54 97 99

119

nanoparticles can be pretreated by chemistry modification and structure modification to increase their surface area, leading to better separation and adsorption capabilities. Moreover, ZIF8 can be incorporated with other materials by being immediately grown on some substrates or being directly mixed with polymers to fabricate membranes for CO 2 adsorption and separation applications. The loading percentage of ZIF-8 among the membrane, type of incorporated materials, and membrane fabrication methods can all affect the CO2 adsorption and separation. Last but not least, the effect of reaction parameters such as temperature, pressure, and the surrounding environment are also summarized in this review. To date, the ZIF-8 nanoparticle-based membranes are highly attractive since this kind of membrane has outstanding benefits for the applications in CO2 adsorption and separation. Given the fact they are promising materials for gas separation which could be applied in many industrial applications and that gas transport mechanisms are still not fully understood, it is still quite challenging to design effective ZIF-8 nanoparticle-based polymeric membranes for a given application condition. The following key issues and challenges still need to be addressed in future research.

10-fold increase in H2/CO2, H2/N2, and H2/CH4 selectivities with respect to 6FDA-durene, which remained 55% of the H2 permeability of 6FDA-durene. Yang et al.119 fabricated MMMs by incorporating ZIF-8 nanoparticles into a polybenzimidazole (PBI) polymer. The experiments of mixed gas tests for H2/CO2 separation were carried out from 35 to 230 °C. Results showed that the membranes exhibited remarkably high H2 permeability and H2/CO2 selectivity. The membrane containing 30 wt % ZIF-8 has an H 2 /CO 2 selectivity of 26.3 with an H 2 permeability of 470.5 Barrer, while the membrane containing 60 wt % ZIF-8 had an H2/CO2 selectivity of 12.3 with an H2 permeability of 2014.8 Barrer. According to the above works, the newly developed membranes may have bright prospects for CO2/H2 separation in realistic industrial applications. Conclusions and Future Perspectives. Table 1 summarized the permeability and selectivity of CO2 by selected ZIF-8based membranes. The incorporated materials, fabrication techniques for the membranes, and special features have been listed for each membrane. In this review, the ZIF-8-based membranes for the applications of CO2 adsorption and separation have been discussed. We respectively summarized four technologies, including pure CO2 adsorption and separations of CO2/CH4, CO2/H2, and CO2/N2. ZIF-8 11209

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based membranes as gas separation membranes can be implemented.

First, new methods and techniques such as the development of facile fabrication methods, scale up technique for industrialization in real life, and increase in the economic effectiveness of the membranes are required to develop and exploit ZIF-8 nanoparticle-based membranes. For example, Chen et al. recently presented a new approach for preparing stable ZIF-8 coatings by a unique hot-pressing method which was both solvent-free and binder-free.123 During the procedure, high temperature and pressure were applied at the same time which produced rapid growth of ZIF-8 nanocrystals onto desired substrates. This continuous production of ZIF-8 membranes using a roll-to-roll machine makes the industrial applications of ZIF-8-based membranes more feasible. Second, the poor distribution and accumulation of ZIF-8 nanoparticles within polymeric membranes are significant problems. At the dispersed phase, ZIF-8 nanoparticles should be well dispersed in the polymeric membranes in order to obtain gas separation membranes with high performance. Therefore, the ideal ZIF-8-ased membranes should have welldispersed inorganic fillers, and the loading is as high as possible. Furthermore, the adhesion between ZIF-8 nanoparticles and polymer materials should be good to avoid imperfect void structures and pinholes. In order to achieve this, the ZIF-8 surface is usually modified with functional groups which can interact with suitable pendent groups of polymers. For instance, good compatibility between ZIF-8 and polymers could be achieved by designing hydrogen bonding and covalent bonding between them. Experimental, computational, and theoretical efforts are all required to address these issues. For example, different theoretical models such as the barrier film model and the Maxwell model should be introduced and/or modified to describe the separation behavior of the membranes which can be beneficial to the design of membranes with high separation performance. Third, environmental parameters such as humidity need to be systematically investigated since the vast majority of gas separation applications are in ambient conditions and the adsorption of moisture is inevitable. This is critical to the reallife application. However, only few research works pay attention to this. Therefore, investigation of the effect of humidity on different gas separation membranes should be carried out since it is a key requirement and essential to the design of applications of gas separation membranes in real life. Fourth, mechanical stability as an important parameter should be considered and addressed for the gas separation applications of ZIF-8-based membranes. To obtain high performance of the gas separation of ZIF-8-based membranes, it is greatly desirable to improve the ZIF-8 loading amount. However, it will result in the loss of mechanical property of the membranes. Therefore, a suitable amount of ZIF-8 should be used during ZIF-8-based membrane fabrication. Moreover, new polymer materials should be synthesized and developed by a variety of methods such as copolymerization, blending, grafting, and cross-linking to overcome shortcomings including swelling, and plasticization. In summary, similar to any gas separation membranes, concerns such as long-term stability under practical conditions (pressure, temperature, and contaminants), chemical modifications, good compatibility, permeability, and plasticization resistance should be addressed in order to make ZIF-8-based membranes for real-life applications. Although significant amounts of works have been done on these topics, there is still a long way to go before large-scale application of ZIF-8-



AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (X. Gong). *E-mail: [email protected] (T. Kuang). ORCID

Xiao Gong: 0000-0002-5513-5354 Notes

The authors declare no competing financial interest. Biographies

Dr. Xiao Gong is currently a Full Professor in the State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, China. He received his Ph.D. in materials science from Zhejiang University in 2009. Prior to joining Wuhan University of Technology, he was a postdoctoral associate in Prof. Lei Li’s group at the University of Pittsburgh, USA. His research interests are on selfassembly and polymeric nanomaterials, polymer thin films, colloid surface and interface chemistry, fabrication and characterization of nanostructured materials, and interfacial behavior of ionic liquid on solid surfaces.

Dr. Yongjin Wang is currently an assistant professor in Dalian Institute of Chemical Physics, Chinese Academy of Sciences. She received her B.S. degree in the department of Chemical Engineering from Dalian University of Technology, China (2010) and her Ph.D. degree in the department of Chemical Engineering from the University of Pittsburgh, USA (2016). Her research interests are colloid surface and interface chemistry. 11210

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Dr. Tairong Kuang received his Ph.D. in Materials Processing Engineering from South China University of Technology, Guangzhou, P. R. China. He is currently a lecturer in the key laboratory of polymer processing engineering of the Ministry of Education, South China University of Technology. The primary focus of his research areas are in synthesis, processing and characterization of advanced polymeric materials, and their functional applications in tissue engineering, energy, gas absorption/separation, biosensors, drug delivery, etc.



ACKNOWLEDGMENTS



REFERENCES

The authors thank the National Natural Science Foundation of China (21774098, 21104069) for financial support. This study is also supported by “the Fundamental Research Funds for the Central Universities (WUT: 2017IVA089)”. We also thank the State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology (SYSJJ2017-01), for the support. T. Kuang acknowledges the support of the National Postdoctoral Program for Innovation Talents (BX201700079).

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DOI: 10.1021/acssuschemeng.7b03613 ACS Sustainable Chem. Eng. 2017, 5, 11204−11214